Therapy delivery method and system for implantable medical devices

ABSTRACT

Recent advancements in power electronics technology have provided opportunities for enhancements to implantable medical device circuits. The enhancements have contributed to increasing circuit miniaturization and increased efficiency in the operation of the implantable medical devices. Stimulation therapy waveforms generated by the circuits include a stepped leading-edge that may be shaped having a varying slope and varying amplitudes associated with each of the segments of the slope. A charging circuit having a single primary transformer winding and a single secondary transformer winding that is coupled to a plurality of capacitors is utilized to generate the therapy stimulation waveforms. The stimulation waveform of the present disclosure may be dynamically shaped as a function of an individual patient&#39;s response. Such stimulation waveforms facilitate achieving lower capture thresholds which reduces the device&#39;s supply consumption thereby increasing longevity of the device and facilitate a reduction of tissue damage.

FIELD

The disclosure relates to body implantable medical devices and, moreparticularly to circuits and techniques for charging capacitors fordelivery of therapy stimulation waveforms.

BACKGROUND

A wide variety of implantable medical devices (IMDs) that employelectronic circuitry for providing various therapies such as electricalstimulation of body tissue, monitoring a physiologic condition, and/orproviding a substance are known in the art. For example, cardiacpacemakers and implantable cardioverter-defibrillators (ICDs) have beendeveloped for maintaining a desired heart rate during episodes ofbradycardia or for applying cardioversion or defibrillation therapies tothe heart upon detection of serious arrhythmias. Other devices deliverdrugs to the brain, muscle and organ tissues, and/or nerves fortreatment of a variety of conditions.

Over the past 20 years, the IMDs have evolved from relatively bulky,crude, and short-lived devices to complex, long-lived, and miniaturizedIMDs that are steadily being miniaturized with their functionalitycontinuously increasing. For example, numerous improvements have beenmade in cardioversion/defibrillation leads and electrodes that haveenabled the cardioversion/defibrillation energy to be preciselydelivered about selected upper and lower heart chambers and therebydramatically reducing the delivered shock energy required to cardiovertor defibrillate the heart chamber. Moreover, the high voltage outputcircuitry has been improved in many respects to provide monophasic,biphasic, or multi-phase cardioversion/defibrillation stimulation (shockor pulse) waveforms that are efficacious, sometimes with particularcombinations of cardioversion/defibrillation electrodes, in lowering therequired shock energy to cardiovert or defibrillate the heart.

The miniaturization of IMDs is driving size and cost reduction of allIMD components including the electronic circuitry components, where itis desirable to reduce the size so that the overall circuitry can bemore compact. The IMDs are powered by an internal power source,typically one or more batteries, that serves a variety of functions,including, but not limited to, supplying power to electronic componentsand circuitry and charging high voltage capacitors that are dischargedthrough medical electrical leads into the heart to regulate heartrhythms. The functional sophistication and complexity of the IMDoperating systems powered by the battery have increased over the years.

Despite the advances, battery powered IMDs must be replaced when thebattery becomes depleted, and therefore conserving battery power remainsimportant to maintain or prolong the life of the IMD. Therefore, as thedimensions of the IMDs decreases, the electronic circuits of the IMDcircuitry are preferred to decrease power consumption in order tomaintain or increase longevity.

SUMMARY

The efforts toward increased miniaturization of implantable medicaldevices have been impeded by the goal of advancing the technologicalcapability of the devices. Often, improvements to the technicalperformance of the device are achieved by complex circuitry thatrequires an increase in the component count—this results in an increasein the overall foot print of device circuitry and power consumption,among other things.

The disclosure describes a charging circuit for an implantable medicaldevice. The charging circuit utilizes a single primary transformerwinding and a single secondary transformer winding that is coupled to aplurality of capacitors. A diode is coupled between the secondarytransformer winding and the plurality of capacitors to maintain apredetermined charging polarity.

In an embodiment, a coupling circuit is provided to dynamicallyconfigure the plurality of capacitors in one of several stackingconfigurations. In one embodiment, the coupling circuit couples theplurality of capacitors to each other in a parallel configuration priorto initiation of the charging.

The charging circuit may be coupled to a power source of the implantablemedical device to draw power for charging the capacitors. An example ofthe power source is a battery that may be non-rechargeable. Theplurality of capacitors may be charged up to a predetermined voltagelevel subsequent to being coupled in the parallel configuration by thecoupling circuit.

In another embodiment, a method for charging a plurality of capacitorsthat are configured to store therapy delivery stimulation energy isdescribed. The method includes coupling the plurality of capacitors in afirst stacking configuration prior to charging the capacitors to apredetermined voltage level. Subsequent to charging, the plurality ofcapacitors are coupled in a second stacking configuration that isdifferent from the first stacking configuration.

In an exemplary embodiment, the plurality of capacitors are coupled inparallel in the first stacking configuration. In an embodiment, theplurality of capacitors are decoupled from each other and successivelystacked in a series configuration by sequentially coupling theindividual capacitors. In another embodiment, one or more of theplurality of capacitors is selectively decoupled and successivelystacked in a series configuration to form a combination series andparallel stacked configuration. A predetermined interval that definesthe duration between the coupling of each individual capacitor may beprovided for the stacking of the capacitors in the series configuration.

BRIEF DESCRIPTION OF THE DRAWINGS

The following drawings are illustrative of particular embodiments of thepresent invention and therefore do not limit the scope of the invention.The drawings are not to scale (unless so stated) and are intended foruse in conjunction with the explanations in the following detaileddescription. Embodiments will hereinafter be described in conjunctionwith the appended drawings wherein like numerals/letters denote likeelements, and:

FIG. 1 is a schematic diagram of a medical device;

FIG. 2 is a schematic diagram of prior art electronic circuitry includedin exemplary medical devices including a conventional charge circuit;

FIG. 3 is a schematic illustrating one embodiment of a charge circuit inaccordance with the present disclosure;

FIG. 4 is an exemplary graph showing results of comparative analysis ofcharging durations for a conventional charge circuit and a chargecircuit in accordance with the present disclosure;

FIGS. 5 and 6 depict therapy delivery circuits of a charge circuit inaccordance with alternate embodiments of the present disclosure;

FIG. 7 depicts an exemplary graph illustrating the amplitude of therapystimulation energy during discharge of the energy stored in a pluralityof output capacitors;

FIG. 8 is a flow chart depicting therapy delivery in accordance with anembodiment of this disclosure; and

FIG. 9 is a flow chart of a method for determining a defibrillationthreshold in accordance with an embodiment of this disclosure.

DETAILED DESCRIPTION

The following detailed description is illustrative in nature and is notintended to limit the embodiments of the invention or the applicationand uses of such embodiments. Furthermore, there is no intention to bebound by any expressed or implied theory presented in the precedingtechnical field, background, brief summary or the following detaileddescription.

In the present disclosure, the inventors have disclosed circuits andtechniques associated with generating therapy stimulation energy havingvarying waveforms for the delivery of therapies such as pacing,defibrillation and cardioversion by an implantable medical device. Theconfigurable waveforms of the therapy stimulation generated inaccordance with aspects of this disclosure include a ramped or steppedleading edge that mimics the cell response behavior for a given patient.The waveforms reduce the thresholds required to achieve capture, therebyincreasing the efficiency and effectiveness of the implantable medicaltherapies.

FIG. 1 is a schematic diagram of an exemplary medical device in whichthe present invention may be usefully practiced. As illustrated in FIG.1, the present invention may be utilized in an implantable medicaldevice 14 that includes a housing 15 containing circuitry for operatingdevice 14 that is subcutaneously implanted in a patient, outside theribcage of patient 12, anterior to the cardiac notch, for example.According to an embodiment, housing 15 may be implanted in the pectoralregion of the patient 12. Further, device 14 may include a subcutaneoussensing and cardioversion/defibrillation therapy delivery lead 18coupled to the device 14 that is tunneled subcutaneously into a locationadjacent to a portion of a latissimus dorsi muscle of patient 12.Specifically, lead 18 is tunneled subcutaneously from the median implantpocket of device 14 laterally and posterially to the patient's back to alocation opposite the heart such that the heart 16 is disposed betweenthe device 14 and the distal electrode coil 24 and distal sensingelectrode 26 of lead 18.

It is understood that while the subcutaneous device 14 is shownpositioned through loose connective tissue between the skin and musclelayer of the patient, the term “subcutaneous device” is intended toinclude a device that can be positioned in the patient to be implantedusing any non-intravenous location of the patient, such as below themuscle layer or within the thoracic cavity, for example.

Further referring to FIG. 1, programmer 20 is shown in telemetriccommunication with device 14 by wireless communication link 22.Communication link 22 may be any appropriate wireless link such asBluetooth, NFC, WiFi, MICS, or as described in U.S. Pat. No. 5,683,432“Adaptive Performance-Optimizing Communication System for Communicatingwith an Implantable Medical Device” to Goedeke, et al.

Device 14 may be constructed from stainless steel, titanium or ceramicas described in U.S. Pat. No. 4,180,078 “Lead Connector for a BodyImplantable Stimulator” to Anderson and U.S. Pat. No. 5,470,345“Implantable Medical Device with Multi-layered Ceramic Enclosure” toHassler, et al. The electronics circuitry of device 14 may beincorporated on a polyamide flex circuit, printed circuit board (PCB),ceramic substrate with integrated circuits packaged in leadless chipcarriers, chip scale packaging, and/or wafer scale packaging.

Lead 18, which is inserted within a connector (not shown) positioned onhousing 15 to electrically couple the lead to the circuitry located inhousing 15, includes a distal defibrillation coil electrode 24, a distalsensing electrode 26, an insulated flexible lead body and a proximalconnector pin (not shown) for connection to housing 15 via theconnector. In some embodiments, distal sensing electrode 26 may be sizedappropriately to match the sensing impedance of one or more electrodes28 that are positioned along housing 15 to form a housing-basedsubcutaneous electrode array with electrodes 28 positioned to formorthogonal signal vectors.

Device 14 in an embodiment of the present invention includesminiaturized circuitry for providing therapy, as described in detailbelow. Optical hemodynamic sensor 17 is preferably a multiple waveformoximeter, such as a pulse oximeter or a mixed-venous oxygen sensor, forexample. Electrodes 28 and optical sensor 17 are welded into place onthe outer surface of the housing 15 and are connected via wires (notshown) to electronic circuitry (described herein below) located insidehousing 15. Electrodes 28 may be constructed of flat plates, oralternatively, spiral electrodes as described in U.S. Pat. No. 6,512,940“Subcutaneous Spiral Electrode for Sensing Electrical Signals of theHeart” to Brabec, et al., and mounted in a non-conductive surroundshroud as described in U.S. Pat. No. 6,522,915 “Surround ShroudConnector and Electrode Housings for a Subcutaneous Electrode Array andLeadless ECGs” to Ceballos, et al and U.S. Pat. No. 6,622,046“Subcutaneous Sensing Feedthrough/Electrode Assembly” to Fraley, et al.

The electronic circuitry employed in device 14 can take any of the knownforms that detect a tachyarrhythmia from the sensed ECG and providecardioversion/defibrillation shocks as well as post-shock pacing asneeded while the heart recovers. An exemplary simplified block diagramof such circuitry adapted to function employing the first and secondcardioversion-defibrillation electrodes as well as the ECG sensing andpacing electrodes described herein below is set forth in U.S. Pat. No.7,647,095, “Method and Apparatus for Verifying a Determined CardiacEvent in a Medical Device Based on Detected Variation in HemodynamicStatus” to Bhunia incorporated herein by reference in its entirety. Itwill be understood that the simplified block diagram does not show allof the conventional components and circuitry of such devices includingdigital clocks and clock lines, low voltage power supply and supplylines for powering the circuits and providing pacing pulses or telemetrycircuits for telemetry transmissions between the device 14 and externalprogrammer 20.

FIG. 2 is a schematic diagram of a prior art electronic circuitry 600included in exemplary medical devices having a conventional capacitorconfiguration and employing a conventional transformer coupling forcharging the capacitors. The electronic circuitry includes software,firmware and hardware that cooperatively monitor the ECG, determine whena cardioversion-defibrillation shock or pacing is necessary, and deliverprescribed cardioversion-defibrillation and pacing therapies. Many ofthe aspects of the prior art electronic circuitry and in particularthose shown in the region defined by the enclosed dashed line 602 maysuitably be used in conjunction with the aspects set forth in theexemplary embodiments of the present disclosure.

As illustrated in FIG. 2, the electronic circuitry 600 includes one ormore power sources such, for example, as a low voltage battery 610 and ahigh voltage battery 612. Low voltage battery 610 powers the devicecircuitry and the pacing output capacitors to supply pacing energy in amanner well known in the art. The low voltage components, such as thoseassociated with pacing, are charged to a pre-programmed voltage level bya low-voltage charging circuit 668 under control of the signal VBATTprovided on line 670. Low voltage supply 668 provides regulated power tothe low voltage ICs, hybrid circuits, and discrete components of theelectronic circuit.

It is understood that although the prior art system of FIG. 2 includesboth low and high power therapy, the present invention may be employedin a device that provides only one therapy, such as a high powerdefibrillation therapy, for example.

In FIG. 2, sense amp 614 in conjunction with pacer/device timing circuit616 processes the far field ECG sense signal that is developed across aparticular ECG sense vector defined by a selected pair of thesubcutaneous electrodes 28 or, optionally, a virtual signal if selected.The selection of the sensing electrode pair is made through the switchmatrix/MUX 618 in a manner to provide the most reliable sensing of theEGM signal of interest, which would be the R wave for patients who arebelieved to be at risk of ventricular fibrillation. The far field ECGsignals are passed through the switch matrix/MUX 618 to the input of thesense amplifier 614 that, in conjunction with pacer/device timingcircuit 616, evaluates the sensed EGM. Bradycardia, or asystole, istypically determined by an escape interval timer within the pacer timingcircuit 616 and/or the control circuit 620. Pace trigger signals areapplied to the pacing pulse generator 622 generating pacing stimulationwhen the interval between successive R-waves exceeds the escapeinterval. Bradycardia pacing is often temporarily provided to maintaincardiac output after delivery of a cardioversion-defibrillation shockthat may cause the heart to slowly beat as it recovers back to normalfunction. Sensing subcutaneous far field signals in the presence ofnoise may be aided by the use of appropriate denial and extensibleaccommodation periods as described in U.S. Pat. No. 6,236,882 “NoiseRejection for Monitoring ECGs” to Lee, et al.

Detection of a malignant tachyarrhythmia is determined in the controlcircuit 620, for example, as a function of the intervals between R-wavesense event signals that are output from the pacer/device timing 616 andsense amplifier circuit 614 to the timing and control circuit 620.Supplemental sensors such as tissue color, tissue oxygenation,respiration, patient activity and the like may be used to contribute tothe decision to apply or withhold a defibrillation therapy as describedgenerally in U.S. Pat. No. 5,464,434 “Medical Interventional DeviceResponsive to Sudden Hemodynamic Change” to Alt. Sensor processing unit662 provides sensor data to microcomputer 624 via data bus 628.

Certain steps in the performance of the detection algorithm criteria arecooperatively performed in microcomputer 624, including microprocessor,RAM and ROM, associated circuitry, and stored detection criteria thatmay be programmed into RAM via a telemetry interface (not shown). Dataand commands are exchanged between microcomputer 624 and timing andcontrol circuit 620, pacer timing/amplifier circuit 616, and highvoltage output circuit 626 via a bi-directional data/control bus 628.The pacer timing/amplifier circuit 616 and the control circuit 620 areclocked at a slow clock rate. The microcomputer 624 is normally asleep,but is awakened and operated by a fast clock by interrupts developed byeach R-wave sense event, on receipt of a downlink telemetry programminginstruction or upon delivery of cardiac pacing pulses to perform anynecessary mathematical calculations, to perform tachycardia andfibrillation detection procedures, and to update the time intervalsmonitored and controlled by the timers in pacer/device timing circuitry616.

The algorithms and functions of the microcomputer 624 and controlcircuit 620 employed and performed in detection of tachyarrhythmias areset forth, for example, in commonly assigned U.S. Pat. No. 5,354,316“Method and Apparatus for Detection and Treatment of Tachycardia andFibrillation” to Keimel; U.S. Pat. No. 5,545,186 “Prioritized Rule BasedMethod and Apparatus for Diagnosis and Treatment of Arrhythmias” toOlson, et al, 5,855,593 “Prioritized Rule Based Method and Apparatus forDiagnosis and Treatment of Arrhythmias” to Olson, et al and 5,193,535“Method and Apparatus for Discrimination of Ventricular Tachycardia fromVentricular Fibrillation and Treatment Thereof” to Bardy, et al.Particular algorithms for detection of ventricular fibrillation andmalignant ventricular tachycardias can be selected from among thecomprehensive algorithms for distinguishing atrial and ventriculartachyarrhythmias from one another and from high rate sinus rhythms thatare set forth in the '316, '186, '593 and '535 patents.

The detection algorithms are highly sensitive and specific for thepresence or absence of life threatening ventricular arrhythmias, e.g.,ventricular tachycardia (VT) and ventricular fibrillation (VF). When amalignant tachycardia is detected, high voltage capacitors 630, 632 and634 are charged to a pre-programmed voltage level by a high-voltagecharging circuit 638. It is generally considered inefficient to maintaina constant charge on the high voltage output capacitors 630, 632 and634. Instead, charging is initiated when control circuit 620 issues ahigh voltage charge command HVCHG delivered on line 640 to high voltagecharge circuit 638 and charging is controlled by means of bi-directionalcontrol/data bus 642 and a feedback signal VCAP from the HV outputcircuit 626. High voltage output capacitors 630, 632 and 634 may be offilm, aluminum electrolytic or wet tantalum construction.

The negative terminal of high voltage battery 612 is directly coupled tocommon ground (Vcc). Switch circuit 644 is normally open so that thepositive terminal of high voltage battery 612 is disconnected from thepositive power input of the high voltage charge circuit 638. The highvoltage charge command HVCHG is also conducted via conductor 646 to thecontrol input of switch circuit 644, and switch circuit 644 closes inresponse to connect positive high voltage battery voltage EXT B+ to thepositive power input of high voltage charge circuit 638. Switch circuit644 may be, for example, a field effect transistor (FET) with itssource-to-drain path interrupting the EXT B+ conductor 646 and its gatereceiving the HVCHG signal on conductor 640. High voltage charge circuit638 is thereby rendered ready to begin charging the high voltage outputcapacitors 630, 632, and 634 with charging current from high voltagebattery 612. In embodiments having both pacing andcardioversion/defibrillation, the circuit may be implemented with ORgate 664 to switch between a LVCHG signal and the HVCHG signal.

High voltage output capacitors 630, 632, and 634 may be charged to veryhigh voltages, e.g., 700-3150V, to be discharged through the body andheart between the electrode pair of subcutaneouscardioversion-defibrillation electrodes 648 and 650. High voltagecapacitors 630, 632 and 634 are charged by high voltage charge circuit638 and a high frequency, high-voltage transformer 652 as described indetail in commonly assigned U.S. Pat. No. 4,548,209 “Energy Converterfor Implantable Cardioverter” to Wielders, et al. Proper chargingpolarities are maintained by diodes 654, 656 and 658 interconnecting themultiple secondary windings of high-voltage transformer 652 respectivelyassociated with the capacitors 630, 632, and 634. As noted above, thestate of capacitor charge is monitored by circuitry within the highvoltage output circuit 626 that provides a VCAP, feedback signalindicative of the voltage to the timing and control circuit 620. Timingand control circuit 620 terminates the high voltage charge command HVCHGwhen the VCAP signal matches the programmed capacitor output voltage,i.e., the cardioversion-defibrillation peak shock voltage.

Control circuit 620 then develops control signal NPULSE 1 that isapplied to the high voltage output circuit 626 for triggering thedelivery of cardioverting or defibrillating shocks. In particular, theNPULSE 1 signal triggers discharge of the capacitor 630, 632 and 634. Inthis way, control circuitry 620 serves to control operation of the highvoltage output stage 626, which delivers high energycardioversion-defibrillation shocks between the pair of thecardioversion-defibrillation electrodes 648 and 650 coupled to the HV-1and COMMON output.

Thus, device 14 monitors the patient's cardiac status and initiates thedelivery of a cardioversion-defibrillation shock through thecardioversion-defibrillation electrodes 648 and 650 in response todetection of a tachyarrhythmia requiring cardioversion-defibrillation.The high HVCHG signal causes the high voltage battery 612 to beconnected through the switch circuit 644 with the high voltage chargecircuit 638 and the charging of output capacitors 630, 632, and 634 tocommence. Charging continues until the programmed charge voltage isreflected by the VCAP signal, at which point control and timing circuit620 sets the HVCHG signal low terminating charging and opening switchcircuit 644. Typically, the charging cycle takes only fifteen to twentyseconds, and occurs very infrequently. The device 14 can be programmedto attempt to deliver cardioversion shocks to the heart in the mannersdescribed above in timed synchrony with a detected R-wave or can beprogrammed or fabricated to deliver defibrillation shocks to the heartin the manners described above without attempting to synchronize thedelivery to a detected R-wave. Episode data related to the detection ofthe tachyarrhythmia and delivery of the cardioversion-defibrillationshock may be stored in RAM for uplink telemetry transmission to anexternal programmer as is well known in the art to facilitate indiagnosis of the patient's cardiac state. A patient receiving the device14 on a prophylactic basis would be instructed to report each suchepisode to the attending physician for further evaluation of thepatient's condition and assessment for the need for implantation of amore sophisticated implantable cardio-defibrillator device (ICD). Inother embodiments, no storage of episode data will take place.

FIG. 3 is a schematic illustrating one embodiment of a charge circuit 40in accordance with the present disclosure. The charge circuit 40 can becoupled to the elements in the region 602 shown in FIG. 2 or similar IMDsystems in accordance with the present invention. As illustrated in FIG.2, the conventional charge circuit 606 includes a transformer 652 with aprimary winding 652 a and multiple secondary windings 652 b, 652 c and652 d. The number of the secondary windings corresponds to the number ofoutput capacitors for holding charge for delivery of therapy by the IMD.In other words, conventional IMD circuits having the output capacitorshardwired in series have required multiple secondary transformerwindings to charge up the capacitors to the desired voltage level.

The inventors have observed that the requirement to provide multiplesecondary transformer windings has translated to transformers that arebulky. Moreover, the supply voltage required to charge the outputcapacitors in the conventional charge circuit must be kept at arelatively high value because of the serial arrangement of the outputcapacitors requiring multiple transformer secondary windings to becoupled to each of the output capacitors. In the conventional chargecircuit, the supply voltage provided is typically on the order of 700 Vto 1000 V. One of the benefits of the reduction in the charge voltage isthat it facilitates a reduction in size as well as the stress placed onthe charging components.

Returning to FIG. 3, charge circuit 40 is illustrated having atransformer 42 including a single primary winding 44 and a singlesecondary winding 46. The single secondary winding 46 is coupled to adelivery circuit through a single diode 48. In accordance with furtheraspects of the present disclosure, the delivery circuit includes aplurality of output capacitors that are controlled by a coupling circuit64 (FIG. 5) and 164 (FIG. 6) that controls the coupling of the pluralityof capacitors. As used in this disclosure, a plurality refers to aninteger number of two or more.

The details of the stacking operation of the coupling circuits aredescribed in further detail in FIGS. 5 and 6. Briefly, the couplingcircuit stacks the output capacitors in a stacking configurationselected from one of multiple stacking configurations including a seriesconfiguration, parallel configuration, or a combination series andparallel configuration during various time periods. For example, theoutput capacitors may be stacked in a parallel configuration during thecharge-up of the output capacitors to the supply voltage level. Prior toor subsequent to the charging of output capacitors, the coupling circuitmay couple the output capacitors in any other desired stackingconfigurations. The charging of the output capacitors may be performedin any known manner, including a “flyback” fashion. One such manner isset forth in commonly assigned U.S. Pat. No. 5,265,588, incorporatedherein by reference in its entirety.

The stacking of the output capacitors in a parallel configuration duringcharging reduces the supply voltage required to charge the capacitors.In the presently described embodiment, the supply voltage required tocharge the output capacitors, can be reduced to about 250 V incomparison to the conventional charge circuit 602. The reduction in thevoltage requirement also facilitates a reduction in the voltage ratingof components associated with the charge circuit 40 as well as areduction in the component count which may improve reliability of theimplantable medical device.

The details of the construction of the transformer are not critical tothe understanding of embodiments of the present disclosure and can be inaccordance with any known methodologies. However, utilizing atransformer 42 having a single primary winding and single secondarywinding as described in accordance with embodiments of the presentinvention, facilitates a reduction in the size of the transformerrequired to charge up the output capacitors. The space otherwiseoccupied by the additional secondary windings of transformers such asthat of the charge circuit 602 and the corresponding additional diodescoupled to the output capacitors can be allocated to other componentsfor added functionality or can facilitate a reduction in the overallfootprint of the charge circuit 40.

FIG. 4 is an exemplary graph showing results of comparative analysis ofcharging durations for a conventional charge circuit and a chargecircuit in accordance with the present disclosure. Graph 54 depicts afirst trace 56 and a second trace 58 that are plotted on an X, Y axischart. The horizontal (X) axis of graph 54 represents the time inseconds and the vertical (Y) axis represents the energy in volts.

The first trace 56, illustrated as a solid line, represents a plot ofthe conventional charge circuit 602 charging up the output capacitorsthat are coupled in series to the supply voltage. The second trace 58,illustrated as dotted line, represents a plot of the charge circuit 40charging up the output capacitors coupled in parallel to the supplyvoltage.

The traces 56, 58 comparatively illustrate the relative charge durationsfor the conventional charge circuit and a charge circuit in accordancewith the present disclosure. In this example, the charge circuit ischarging to a reduced energy level as a result of the reduced thresholdrequirements, which is facilitated by the delivery of a modifiedstimulation waveform in accordance with the present disclosure. As shownin graph 54, the charge circuit 40 takes a comparatively shorter time tocharge up the output capacitors to the supply voltage relative to thetime taken by the conventional charge circuit 602.

FIGS. 5 and 6 depict therapy delivery circuits 60 a, 60 b (collectively“60”) in accordance with alternate embodiments of the presentdisclosure. The exemplary therapy delivery circuits 60 generate therapystimulation having dynamically configurable waveforms that may becustomized based on the patient's physiological response. Such therapystimulation waveforms may be generated having a stepped leading-edge.

As previously discussed in FIG. 3, the delivery circuit 60 is coupled tothe charge circuit 40 via diode 48. Delivery circuit 60 includes aplurality of output capacitors 62 a, 62 b, and 62 c, (collectively“output capacitors 62”). The output capacitors 62 are charged by thecharge circuit 40 and hold the energy for generating therapy stimulationwaveforms.

In accordance with the present disclosure, the stacking of the outputcapacitors 62 relative to one another is dynamically configurable. Atleast one of the output capacitors 62 is coupled to the couplingcircuit. The coupling circuit dynamically configures the outputcapacitors to form various parallel and series combinations.

FIG. 5 describes an embodiment in which delivery of therapy is initiatedwith the plurality of output capacitors 62 being coupled in parallel.This embodiment may be utilized to induce ventricular fibrillation in apatient during implantation to facilitate defibrillation thresholdtesting. The output capacitors may subsequently be reconfigured inaccordance with the dynamic reconfiguration techniques of the presentdisclosure for therapy delivery. For ease of reference, the descriptionof FIG. 5 is discussed in conjunction with the circuit diagramsillustrated in FIGS. 5A-5C. Each of FIGS. 5A-5C depicts the functionalequivalent circuit subsequent to the coupling of the output capacitorsfor different phases of therapy delivery in accordance with theembodiment of FIG. 5.

Turning to the embodiment of FIG. 5, a first switch, such as triac 66 a,has a first terminal that is coupled to the negative terminal ofcapacitor 62 b and a second terminal of triac 66 a is coupled to thepositive terminal of capacitor 62 a, the cathode of diode 48 and ananode of diode 68 a. The coupling circuit 64 also includes a secondtriac 66 b that is coupled at a first terminal to the negative terminalof capacitor 62 c and at a second terminal between the positive terminalof the capacitor 62 b, the cathode of diode 68 a and an anode of diode68 b. The diodes 68 a, 68 b maintain the discharge polarity by settingthe direction for flow of charge of the therapy stimulation energy.

The coupling circuit 64 further includes a delivery control module 70.The gates of triacs 66 a, 66 b are coupled to the delivery controlmodule 70 through gate triggered field effect transistors (FETs) 72 a,72 b respectively. The delivery control module 70 activates ordeactivates the FETs 72 (switch to an “ON” or “OFF” position,respectively) to control the stacking configuration of the outputcapacitors 62. The selective activation or deactivation of the FETs 72will switch the triacs 66 ON or OFF (activate or deactivate),respectively. The delivery control module 70 may further be coupled toFETs 74 a, 74 b that connect the output capacitors 62 b and 62 c,respectively, to the common ground (Vcc). Diodes 76 a and 76 b,illustrated in phantom lines, represent the effective intrinsic bodydiodes of FETs 74 a and 74 b respectively.

In one embodiment, the output capacitors 62 may be stacked in a firstconfiguration such that all the capacitors are coupled in parallel.Stacking of the output capacitors 62 in the parallel configuration isachieved by turning FETs 740N and by turning triacs 66 OFF. The outputcapacitors 62 may then be charged in this parallel configuration to apredetermined voltage level. As described in FIG. 3, the chargingpolarity (i.e., the direction for the flow of current from the chargecircuit 40 to the output capacitors 62) is set by diode 48. The outputcapacitor 62 a is charged via the diode 48 with a connection to commonground. Output capacitor 62 b is charged through the diode 48, diode 68a, through FET 74 a with a connection to common ground. Output capacitor62 c is charged through the diode 48, diode 68 a, diode 68 b, throughFET 74 b with a connection to common ground. In some embodiments, aminimal voltage drop relative to the overall charge voltage may beobserved for output capacitors 62 b, 62 c due to the charging throughdiodes 68. However, such voltage difference is analogous to the voltagedifference arising from the mismatch in the individual secondarytransformer windings for each capacitor in the conventional circuit.

Subsequent to a confirmation that the output capacitors 62 are fullycharged, the coupling circuit 64 configures the output capacitors 62 ina desired stacking configuration to initiate the delivery of therapy.For example, the output capacitors 62 may remain in the firstconfiguration or the configuration may be changed from the firstconfiguration to a second configuration that is different from thefirst. Even further, the present disclosure facilitates the dynamicreconfiguration of the output capacitors 62 from the second to a thirdand, even further, to other subsequent configurations during therapydelivery. As such, the output capacitors 62 are configured in a givenone of multiple configurations prior to or, during therapy delivery. Thedischarge of energy by the output capacitors 62 in any givenconfiguration coincides with a distinct therapy delivery phase.

The therapy is delivered through electrodes that may be coupled to thedelivery circuit 60 through a delivery bridge, provided in HV out 626,for example. The operation of the delivery bridge does not impact theoperation of the present invention and is therefore not shown ordiscussed. In essence, the delivery bridge comprises switches thatconfigure a delivery path that includes the electrodes. In other words,the delivery bridge directs flow of therapy stimulation energy to theheart and back to the IMD in a unipolar or bipolar electrodeconfiguration as is described in detail in published literature. Thetherapy stimulation energy may be discharged from the output capacitorsthrough the electrodes in multiple phases such that capacitor 62 adischarges through diodes 68 a, 68 b; capacitor 62 b discharges throughdiode 68 b; and capacitor 62 c discharges directly via the deliverybridge.

In one embodiment, therapy delivery is initiated with the outputcapacitors 62 remaining in the first configuration. In other words, theoutput capacitors 62 a, 62 b, and 62 c are stacked in a parallelconfiguration for a predetermined interval of time. In this first phase,the output capacitors 62 a, 62 b, and 62 c are decoupled from the chargecircuit and coupled to the electrodes (not shown) to initiate dischargeof the capacitors for delivery of a first phase of the therapy. FIG. 5Adepicts a functional equivalent circuit of the stacking of outputcapacitors 62 in the first configuration. As is shown in FIG. 5A, theoutput capacitors 62 are coupled in parallel and the common terminals ofthe capacitors connect to the delivery bridge. The equivalent circuit ofFIG. 5A is achieved by configuring the FETs 74 in an ON position and byturning OFF the triacs 66.

As stated above, the coupling of the output capacitors 62 may bereconfigured from the first configuration to a second configurationfollowing discharge of the output capacitors during the duration of thefirst predetermined interval. It is contemplated that the duration oftime from initiation of discharge to the reconfiguration of the outputcapacitor coupling may be selectively varied to provide a desiredwaveform. Further, the waveform may be shaped by varying theconfiguration of the stacking of capacitors for each interval, keepingin mind that the output capacitors may be reconfigured individually orin any combination to achieve a stacking configuration for the second,third, and subsequent therapy delivery phases.

Continuing with the illustrative embodiment, the stacking of one of thecapacitors, such as output capacitor 62 c may be reconfigured for thesecond configuration. The reconfiguration couples the output capacitor62 c in series with the parallel combination of capacitors 62 a and 62 bafter an interval (first predetermined interval) following theinitiation of the discharge of part of the energy in the capacitors 62a, 62 b, and 62 c in the first phase. The equivalent circuit for thesecond configuration is depicted in FIG. 5B. The series coupling ofcapacitor 62 c to the parallel coupled capacitors 62 a and 62 b isperformed by turning triac 66 b to the ON position while turning FET 74b OFF. The reconfiguration of the stacking of the output capacitors 62a, 62 b and 62 c to couple capacitor 62 c in series initiates the secondphase of the therapy delivery. The first predetermined interval isdefined to correspond with a point in time during the discharge of theoutput capacitors 62 that will provide a desired ramp profile for thestimulation waveform. Turning OFF the FET 74 b prevents shorting throughground in response to turning triac 66 b ON to couple output capacitor62 c in series with the parallel combination of output capacitors 62 a,62 b.

In the illustrative embodiment, some of the energy in output capacitors62 is discharged during a second predetermined interval for a secondphase of the therapy delivery. The capacitors may again be reconfiguredinto a third configuration that provides for stacking all three of theoutput capacitors in series. In particular, output capacitor 62 b isplaced in series with output capacitor 62 a. The equivalent circuit forthe third configuration is depicted in FIG. 5C. The series coupling ofcapacitor 62 a to capacitor 62 b is performed by turning triac 66 a tothe ON position while turning FET 74 a OFF. Triac 66 b remains in the ONposition and FET 74 b remains in the OFF position.

FIG. 6 depicts an alternative embodiment of a delivery circuit. Thedelivery of therapy in accordance with the embodiment of FIG. 6 isinitiated with discharge of a first, single, capacitor. The other outputcapacitors of the plurality of capacitors may subsequently be coupled inseries with the first capacitor in accordance with the dynamicreconfiguration techniques of the present disclosure for therapydelivery.

For ease of reference, the description of FIG. 6 is discussed inconjunction with the circuit diagrams illustrated in FIGS. 6A-6E. Eachof the circuits in FIGS. 6A-6E depict the functional equivalent of thecoupling of the output capacitors for different phases of therapydelivery in accordance with the embodiment of FIG. 6. Furthermore, theelements of delivery circuit 60B of FIG. 6 corresponding to those ofdelivery circuit 60 a in FIG. 5 are numbered with identical referencedesignators. The reader is referred to the preceding description of FIG.5 for a full discussion pertaining to those components.

The delivery circuit 60 b of FIG. 6 includes a triac 174 a that iscoupled between output capacitor 62 b and ground. A diode 176 a iscoupled in parallel with triac 174 a with the diode 176 a cathode isconnected to ground. A triac 174 c is further coupled between outputcapacitor 62 a and ground. A diode 176 c is coupled in parallel withtriac 174 c and the diode 176 c cathode is connected to ground. Bothtriacs 174 a and 174 c are connected to the delivery control module 70.

Although it is not depicted as such in the illustrative embodiment,alternative embodiments may include reconfiguration of the terminals ofthe triacs 174 a, 174 c with or without additional protection circuitryfor connecting the triacs to the delivery control module 70. Suchprotection circuitry may include FETs with the coupling being similar tothat of the coupling of triacs 66.

In the embodiment of FIG. 6, a first phase of therapy delivery isinitiated with only a single one of the output capacitors delivering theenergy for the stimulation therapy. In this embodiment, the outputcapacitors 62 are not stacked during the first phase; that is, alltriacs 66 a, 66 b, 174 a and 174 c are set in the OFF position. Outputcapacitor 62 c is coupled to the electrodes (not shown) for discharge ofenergy during the first phase. The equivalent circuit of outputcapacitors 62 is depicted in FIG. 6A. The duration over which the energyis discharged may be predetermined such that only part of the chargestored in the output capacitor 62 c is discharged during a firstinterval.

At the expiration of the first interval, a second of the outputcapacitors, e.g., 62 b may be stacked in series with the first capacitor62 c for a second phase of the therapy delivery. The representativeequivalent alternative circuits following the coupling of outputcapacitor 62 b in series to capacitor 62 c are depicted in FIGS. 6B and6C. In the second phase, capacitor 62 c may be stacked on top ofcapacitor 62 b by turning FET 74 b OFF and triacs 66 b and 174 a ON.

The duration of the first predetermined interval may be set tocorrespond with a point in time during the discharge of the outputcapacitor 62 c to provide a desired ramp profile for the stepped leadingedge of the stimulation waveform.

A second predetermined interval may be provided to define the durationduring which some of the energy in the serially coupled capacitors 62 band 62 c is discharged. The duration of the second predeterminedinterval may be measured beginning when the second capacitor, capacitor62 b in this example, is coupled to the first capacitor.

Following the expiration of the second predetermined interval, a thirdcapacitor, capacitor 62 a in this example, is connected in series to theserially coupled capacitors 62 b and 62 c. The coupling of capacitor 62a initiates a third phase of the therapy delivery.

A simulation of the therapy stimulation waveform generated duringdischarge of the output capacitors 62 in the exemplary first, second andthird phases of FIG. 6 is described in conjunction with FIG. 7.

In the illustrative embodiment of FIG. 6, the series coupling ofcapacitor 62 a to the serially connected capacitors 62 c and 62 b mayperformed by turning triac 174 a OFF, while turning triac 66 a ON andtriac 174 c ON. Triac 66 b remains in the ON position. Thisimplementation stacks the combination of output capacitors 62 c and 62 bon top of output capacitor 62 a. FIGS. 6D and 6E depict the alternativeequivalent circuits for the output capacitors following the seriescoupling of capacitor 62 a to the serially connected capacitors 62 b and62 c.

In other implementations, the delivery of therapy may be initiated inthe first phase with discharge of a different first output capacitor(other than 62 c). While not illustrated, such alternativeimplementations may also include additional components and/or thedelivery control module 70 may be programmed to provide differentcontrol signals.

In the exemplary embodiment, the delivery control module 70 isprogrammed with the durations for the first and second intervals thatcontrol the point in time during the delivery of the therapy stimulationenergy when the output capacitors' stacking is reconfigured. Theselective control over the first and second intervals enables thedischarge profile of the resulting stimulation waveform generated fromthe energy stored in the output capacitors 62 to be manipulated toemulate the heart cell response time for a given patient and therebyprovide efficient threshold levels to ensure capture. Therefore, thedynamic configurability of the stacking of output capacitors 62 coupledwith the control of the coupling intervals facilitates generation ofstimulation waveforms of varying profiles that can be modified tooptimize the therapy based on the heart cell response for differentpatient populations.

The exemplary coupling circuits of FIGS. 5 and 6 include variousswitching components that are dynamically controlled to couple theoutput capacitors in the various stacking configurations. In theillustrative embodiment, the switching components facilitate in thecoupling of the three output capacitors to which they are connected toform the various stacking configurations discussed above. The switchingcomponents may be grouped such that the components associated with thestacking of each capacitor relative to the other capacitors are definedas a stacking circuit module set. For example, a first set in FIG. 5 mayinclude triac 66 a, gate trigger FET 72 a, discharge FET 74 a while asecond set may include triac 66 b, gate trigger FET 72 b, and dischargeFET 74 b. As can be deduced from the illustrations of FIGS. 5 and 6, thenumber of stacking circuit module sets is one less than the number ofcapacitors in the delivery circuit.

In alternative embodiments, a different number of output capacitors 62may be provided to define different stimulation waveforms with differentdelivered energy, slopes or ramp profiles. In such embodiments,additional sets of stacking circuit modules may be provided, asdescribed above, may be included in the coupling circuits 64, 164 todynamically control the stacking of the additional capacitors during thecharging and discharging operations of such additional outputcapacitors.

The therapy delivery in accordance with the present disclosurefacilitates generation of a therapy stimulation waveform that may beshaped based on the patient's physiological response to the stimulationwaveform. Unlike the conventional waveform delivered by the conventionaltherapy delivery circuit which is based on the behavior of the outputcapacitors (i.e., i=C(dV/dt)), the stimulation waveform of the presentdisclosure may be dynamically shaped as a function of an individualpatient's response. In so doing, lower thresholds may be achieved whichreduces the consumption of the device's power supply thereby promotingincreased longevity of the device.

FIG. 7 depicts an exemplary graph 200 illustrating the amplitude oftherapy stimulation energy during discharge of the energy stored in aplurality of output capacitors. The horizontal (X) axis of the graph 200represents time in seconds and the vertical (Y) axis representsstimulation amplitude in volts. The stimulation amplitude is the energydischarged from the output capacitors during delivery of a therapy to apatient.

A first trace 202, shown as a solid line, represents the therapystimulation waveform generated by a conventional delivery circuit 600where three capacitors are coupled, typically hardwired, in series fromthe beginning of the delivery of the therapy until the stimulation pulseis delivered as desired for the given therapy. The trace 202 illustratesthe therapy stimulation waveform as a truncated exponential waveformhaving a nearly instantaneous inclined leading-edge from zero volts tothe maximum amplitude, 1400 volts in this case, with a gradual decayover a period of approximately 4 ms. This trace 202 is representative ofthe profile of a conventional therapy stimulation waveform and is basedon the behavior of the output capacitors (i.e., I=C(dV/dt)), where “i”is the instantaneous current through the capacitor, “C” is thecapacitance of the capacitors in Farads, and dV/dt is the instantaneousrate of voltage change in volts per second.

Trace 204, shown as a dotted line, is an exemplary therapy stimulationwaveform that may be generated by the therapy delivery circuitsdescribed in accordance with aspects of the present disclosure. Inparticular, the waveform corresponds to the discharge of energy in theoutput capacitors 62 during the three phases described in conjunctionwith the circuit of FIG. 6. The trace 204 is illustrated as astimulation therapy waveform having a stepped or ramped leading edge.Unlike the truncated exponential waveform of trace 202, the rampedleading edge of the waveform represented by trace 204 has been observedto more closely mimic a profile of the cardiac cell response time. Inthe case of cardiac cells, for example, the capacity for cells torespond to stimulation energy gradually increases over a period of timeleading to capture. As such, an optimal therapy stimulation waveform isone that increases gradually over a period of time, with the stimulationenergy increasing from zero to the maximum amplitude that achievescapture, 1200 volts in this case. This waveform 204 is more efficientthan the conventional waveform 202 in that the stimulation energy over agiven period of time is reduced.

The exemplary trace 204 is depicted having a first step 206 a and asecond step 206 b to define three segments “A”, “B”, and “C”. The firstsegment A coincides with the discharge of a first of the three outputcapacitors during the first phase of the therapy delivery. In the secondsegment B, a second of the three output capacitors is coupled in seriesto the first capacitor. Therefore, the second segment coincides with thedischarge of the first and second capacitors coupled in series duringthe second phase of therapy delivery. In the third segment C, a thirdcapacitor is coupled in series to the serially coupled first and secondcapacitors so that all three capacitors are coupled in series. Thisthird segment coincides with the discharge of the output capacitors inthe third phase.

In the illustrative embodiment, the time interval between the seriescoupling of the second capacitor to the first capacitor and the seriescoupling of the third capacitor to the serially connected first andsecond capacitors is depicted along the X axis. As is illustrated in thegraph 200, the first and second intervals determine the magnitudes ofeach of the segments A, B, and C of the stepped leading edge of trace204. The slope of trace 204 may also be manipulated based on theduration of the first and second intervals.

The amount of energy delivered to the patient by the conventionalcircuit can be calculated by determining the area under the curve oftrace 202 whereas the amount of energy delivered to the patient inaccordance with aspects of the present disclosure can be calculated bydetermining the area under the curve of trace 204. As is illustrated inthe graph 200, the area under the curve of trace 202 is greater than thearea under the curve of trace 204. This illustration shows that thestimulation energy required to achieve capture when therapy is deliveredthrough the conventional circuit (202) is relatively larger incomparison to the stimulation energy required to achieve capture whendelivered through the delivery circuit of the present disclosure (204).

The inventors have theorized that one of the underlying reasons for thedecrease in the maximum amplitude arises from the ability to generate astimulation waveform that mimics the cell response profile. Because thestimulation waveform is adapted to stimulate the cells during optimaltimes, an excess amount of energy does not have to be provided tocompensate for the delayed cell response.

Therefore, the delivery circuit of the present disclosure exhibits adecrease in the stimulation energy that is delivered through the tissueof the patient. In addition to the significantly lower amplitudediscussed above, the reduced stimulation energy facilitates a reductionin tissue trauma to the patient in comparison to the conventionaldelivery circuit. Moreover, relatively less energy is consumed fordelivery of therapy by the delivery circuit of the present disclosurebecause of the reduced stimulation energy which also means that thecapacitors can be charged up to the full power supply faster in relationto the conventional delivery circuit. The decreased consumption promotesan increase in the response time to cardiac conditions requiringstimulation therapy. The reduction in the energy requirement fordelivery of stimulation therapy also promotes an increase in the devicelongevity.

FIG. 8 is a flow chart depicting therapy delivery tasks in accordancewith an embodiment of this disclosure. For illustrative purposes, thefollowing description of the process in FIG. 8 may refer to elementsmentioned above in connection with FIGS. 3 to 7. In practice, portionsof the process may be performed by different elements of the describedsystem; e.g., implanted sensors, an IMD, or an external monitoringdevice.

The flow chart in FIG. 8 describes the delivery of a therapy regimen,such as pacing, defibrillation, or cardioverting stimulation that maytake the form of a stimulation pulse, a continuous waveform, or thelike, from energy stored in the output capacitors. If therapy is notcurrently enabled, therapy can be initiated by a clinician, the patient,or the device (220). Finally, the device may automatically initiatetherapy based on preprogrammed time of day or due to sensor signals,including electrograms, hemodynamic, activity sensor signals, and otherphysiologic sensor signals. In particular, a signal sensed by the devicesensors may be evaluated to determine whether to initiate therapydeliver.

At task 222, the output capacitors that are configured to hold thecharge for delivery of the therapy are monitored to determine whethersufficient energy is available for delivery of therapy. If it isdetermined that the capacitors are not charged up to the appropriatelevel, the charging circuit is operated to supply the power required tocharge up the capacitor to the appropriate level (224).

Upon confirmation that the capacitors are charged up to the desiredlevel, the delivery circuit initiates the discharge of the outputcapacitors in accordance with the programmed parameters. In an exemplaryembodiment, a first output capacitor is coupled to the delivery bridgeto initiate a first phase of therapy stimulation delivery through theselected electrode (226). The delivery control circuit may be programmedto determine the duration over which the first output capacitor isdischarged. The delivery circuit discharges the given output capacitorthrough the delivery bridge for the programmed duration to deliver thegiven phase of the therapy stimulation (228).

Prior to, or during delivery of therapy in the first phase, the deliverycontrol circuit determines whether additional phases are necessary forthe given therapy regimen (230). For purposes of illustration, thetherapy delivery may be preprogrammed to be provided in three phases inan exemplary embodiment.

The timing of the coupling of the subsequent capacitors to stack eachindividual subsequent capacitor to the previously discharging capacitorin a series configuration may be preprogrammed with the instructionsbeing carried out by the delivery control circuit (232). In theexemplary embodiment, the delivery circuit couples a subsequentcapacitor in series with the capacitor that has previously been coupledto the delivery bridge to initiate the next phase of the delivery oftherapy (234).

The discharge of the series-coupled capacitors is performed for thesubsequent phase of the therapy delivery. FIG. 8 depicts task 234leading back to task 228; this loop represents the number of deliveryphases that may be built in to the delivery of the therapy. Eachsuccessive coupling and discharge of the individual capacitors resultsin a stimulus pulse having a stepped leading edge waveform. Theresulting waveform is a coarse ramp where the number of capacitorsdetermines the number of steps and the slope of the leading edge isdetermined by the timing of the interval between stacking of the outputcapacitors (i.e. step delay). In response to determining that therequisite number of phases has been delivered, and that the therapy hasbeen provided in accordance with preprogrammed parameters the therapydelivery may be terminated in any desired manner such as by decouplingthe output capacitors from the delivery bridge.

The dynamic configurability of the stacking of the output capacitors 62and the intervals between successive coupling of the individualcapacitors 62 provides for manipulation of the waveform profile of theramped leading edge. The slope of the ramped leading edge may also becontrolled by the programmability of the intervals between thesuccessive coupling of the individual output capacitors. For example,the slope of a first ramped leading edge is less steep in animplementation in which the coupling of the output capacitors 62 isperformed following larger intervals compared to the slope of a secondramped leading edge for an implementation in which the coupling of theoutput capacitors 62 is performed following smaller intervals.

In comparison to the truncated exponential waveforms, the ramp waveformsgenerated in accordance with aspects of the present disclosure hasexhibited reductions in the energy required for capture. The reductionshave been in excess of 25%. The inventors have theorized that a leadingfactor for the reduction in threshold energies is the response time forthe tissue cells to which the therapy is delivered. By deliveringtherapy in the form of a waveform that mimics the cell response time, anoptimal amount of energy can be provided to the cells at the optimalresponse time.

FIG. 9 is a flow chart of a method for determining a defibrillationthreshold in accordance with an embodiment of this disclosure. Themethod may utilize devices described in accordance with embodiments ofthis disclosure to induce fibrillation and to subsequently determine athreshold amount of energy for terminating the fibrillation.

At task 302, pacing pulses may be delivered to the patient's heart at adesired pacing pulse energy and desired pacing rate. The pacing pulsesmay be delivered by device 14 or any other device capable of generatingthe pacing pulses.

During the pacing, the electrical conduction of the patient's heart maybe monitored to obtain cardiac signals indicative of the heart'selectrical conduction (304). The cardiac signals are evaluated toidentify the occurrence of the T-wave and, in particular, the ascendingpart of the T-wave (306).

The method determines whether the therapy delivery source is enabled soas to initiate delivery of the energy stored by the output capacitors 62for generating a stimulation pulse (308). Among other things, thetherapy delivery is disabled if the output capacitors are not adequatelycharged. As such, the output capacitors may be charged to store theenergy that is delivered to generate the stimulation pulse (310). Acharge signal may be provided that indicates that the output capacitorshave been charged up to a predetermined level.

The present disclosure facilitates the dynamic configuration of thecapacitors 62 in any desired stacking arrangement. In embodiments inwhich the capacitors are stacked in a series configuration duringcharging, the output capacitors are stacked in a parallel configurationfor delivery of the stimulation pulse to induce ventricularfibrillation. A determination may be made as to whether the stackingconfiguration of the plurality of capacitors is a parallel configuration(312). If not, the capacitors are subsequently stacked in parallel(314).

The method may be configured to provide a desired number of pacingpulses for synchronizing a stimulation pulse prior to delivery of thestimulation pulse. The stimulation pulse may be in the form of a lowenergy or sub-threshold shock. In embodiments utilizing a pacing sourceother than device 14, the pacing source is disconnected, based on aspecified number of delivered pulses, a predetermined time delay orother timing mechanism, prior to delivery of the stimulation pulse.

In response to identifying the occurrence of the T-wave, the stimulationpulse is delivered to the patient coinciding with the ascending part ofthe T-wave (316).

At task 318, the patient's electrical activity is evaluated to determinewhether the delivered stimulation pulse successfully induced ventricularfibrillation. Onset of the ventricular fibrillation may be identifiedbased on the patient's cardiac signals following the stimulation pulsedelivery. If the initial stimulation pulse fails to induce ventricularfibrillation, tasks 302-318 may be repeated for as many iterations asnecessary, with a different amount of energy in each iteration, untilthe ventricular fibrillation is induced.

Following successful induction of the ventricular fibrillation, theoutput capacitors may be recharged and reconfigured for determination ofthe threshold amount of energy that terminates the ventricularfibrillation (320). The charging and configuration of the outputcapacitors for delivery of the defibrillation energy may correspond totechniques described elsewhere in this disclosure.

The flow charts, techniques and technologies presented herein areintended to illustrate the functional operation of an exemplary device,and should not be construed as reflective of a specific form ofsoftware, firmware or hardware necessary to practice the invention. Itis believed that the particular form of software, firmware, and hardwarewill be determined by the particular system architecture employed in theexternal pacing source, external shock source and interface and by thestimulation therapy (pacing and shock) delivery methodologies employedby external sources. For example, an embodiment of a system or acomponent may employ various integrated circuit components, e.g., memoryelements, digital signal processing elements, logic elements, look-uptables, or the like, which may carry out a variety of functions underthe control of one or more microprocessors or other control devices. Tothe extent that there is any ambiguity or inconsistency between the textand the circuit symbols depicted in the figures, the figures will bedeemed to control.

Providing software, firmware and hardware to accomplish the presentinvention, given the disclosure herein, is within the abilities of oneof skill in the art. For the sake of brevity, conventional techniquesrelated to ventricular/atrial pressure sensing, IMD signal processing,telemetry, and other functional aspects of the systems (and theindividual operating components of the systems) may not be described indetail herein. The methods described in conjunction with flow charts maybe implemented in a computer-readable medium that includes instructionsfor causing a programmable processor to carry out the methods described.A “computer-readable medium” includes but is not limited to any volatileor non-volatile media, such as a RAM, ROM, CD-ROM, NVRAM, EEPROM, flashmemory, and the like. The instructions may be implemented as one or moresoftware modules, which may be executed by themselves or in combinationwith other software.

The connecting lines shown in the various figures contained herein areintended to represent example functional relationships and/or physicalcouplings between the various elements. It should be noted that manyalternative or additional functional relationships or physicalconnections may be present in an embodiment of the subject matter.Furthermore, it should be appreciated that the processes may include anynumber of additional or alternative tasks, the tasks shown in FIGS. 8and 9 need not be performed in the illustrated order, and the processmay be incorporated into a more comprehensive procedure or processhaving additional functionality not described herein.

The description refers to elements or nodes or features being“connected” or “coupled” together. As used herein, unless expresslystated otherwise, “connected” means that one element/node/feature isdirectly joined to (or directly communicates with) anotherelement/node/feature, and not necessarily mechanically. Likewise, unlessexpressly stated otherwise, “coupled” means that oneelement/node/feature is directly or indirectly joined to (or directly orindirectly communicates with) another element/node/feature, and notnecessarily mechanically. Thus, although the schematics shown in thefigures depict exemplary arrangements of elements, additionalintervening elements, devices, features, or components may be present inan embodiment of the depicted subject matter.

While the disclosure is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and are herein described in detail. It shouldbe understood, however, that the description herein of specificembodiments is not intended to limit the disclosure to the particularforms disclosed, but on the contrary, the intention is to cover allmodifications, equivalents, and alternatives falling within the spiritand scope of the disclosure as defined by the appended claims.

1. An implantable medical device, comprising: a power source; aplurality of output capacitors; a charge circuit coupled to the powersource and to the plurality of output capacitors, having: a transformerhaving a single primary winding coupled to the power source and a singlesecondary winding coupled to the plurality of output capacitors, each ofthe plurality of output capacitors being coupled to the single secondarywinding during charging; a diode for regulating a charging polarity forthe plurality of output capacitors; a coupling means for coupling theplurality of output capacitors in a parallel configuration prior tocommencement of the charging of the capacitors and in a combinationseries configuration in response to termination of charging of thecapacitors.
 2. The implantable medical device of claim 1, wherein thecoupling means includes: a control module; a plurality of stackingcircuit modules coupled to the control module and operable todynamically configure the coupling of the plurality of capacitors intothe parallel configuration during the charging and into the combinationseries configuration in response to termination of the charging, eachstacking circuit module including: a discharge FET and a switch coupledto a first terminal of one of the plurality of capacitors; and a gatetrigger FET coupled between the control module and the switch forcontrolling the selective activation and deactivation of the switch. 3.The implantable medical device of claim 2, wherein the discharge FET ofeach stacking circuit module is activated and the switch of eachstacking circuit module is deactivated during the charging operation ofthe output capacitors.
 4. The implantable medical device of claim 2,further comprising a diode for biasing a direction current flow throughthe discharge FET.
 5. The implantable medical device of claim 4, whereinthe number of the plurality of stacking circuit modules corresponds tothe number in the plurality of capacitors minus one.
 6. The implantablemedical device of claim 1, further comprising: an input circuit forsensing cardiac signals; a microprocessor programmed to process thesensed signals to detect a predetermined event in response to the sensedsignals; and a controller for controlling charging of the plurality ofoutput capacitors triggered by detection of the predetermined event. 7.The implantable medical device of claim 1, wherein the plurality ofcapacitors are charged to a voltage of approximately 225 volts toapproximately 270 volts.
 8. The implantable medical device of claim 1,wherein the coupling means decouples the plurality of capacitorsimmediately subsequent to termination of the charging prior to stackingthe plurality of capacitors in a combination series configuration. 9.The implantable medical device of claim 1, further comprising a deliverybridge, wherein the plurality of capacitors are coupled in one ofmultiple dynamically-alterable configurations to the delivery bridge fordischarge of the energy stored in the plurality of capacitors.
 10. Theimplantable medical device of claim 9, wherein the discharge of theplurality of capacitors is initiated by coupling a first of theplurality of capacitors to the electrode, and subsequently coupling eachof the other plurality of capacitors to the coupled capacitor in aserial configuration.
 11. An implantable medical system comprising: animplantable medical lead having an electrode; an implantable medicaldevice coupled to the implantable medical lead, the implantable medicaldevice including: a therapy delivery circuit having: a plurality ofcapacitors; means for stacking the plurality of capacitors, wherein themeans for stacking the plurality of capacitors dynamically configuresthe coupling between the capacitors into a first stacked configurationduring a first interval and into a second stacked configuration during asecond interval; a power source; a charge circuit coupled between thepower source and the therapy delivery circuit, wherein the chargecircuit is operable to store energy for delivery of therapy in theplurality of capacitors; and a control module for controlling deliveryof therapy from the therapy delivery circuit to the electrode.
 12. Theimplantable medical system of claim 11, wherein the first intervalcorresponds to a duration during which the plurality of capacitors arebeing charged by the charging circuit, and each of the plurality ofoutput capacitors is simultaneously coupled to the charging circuitduring the charging.
 13. The implantable medical system of claim 11,wherein the second interval corresponds to a duration during which theplurality of capacitors are discharged for delivery of therapy.
 14. Animplantable medical system comprising: an implantable medical leadhaving an electrode; an implantable medical device coupled to theimplantable medical lead, the implantable medical device including: apower source; a therapy delivery circuit having: a plurality ofcapacitors; means for stacking the plurality of capacitors, wherein themeans for stacking the plurality of capacitors dynamically configuresthe coupling between the capacitors into a first stacked configurationduring a first interval and into a second stacked configuration during asecond interval; a charge circuit coupled between the power source andthe therapy delivery circuit, wherein the charge circuit is operable tostore energy for delivery of therapy in the plurality of capacitors; anda control module for controlling delivery of therapy from the therapydelivery circuit to the electrode, wherein the control module controlsthe discharge of energy from the plurality of capacitors to generate astimulation waveform having a stepped leading-edge.
 15. An implantablemedical device, comprising: a power source; a plurality of outputcapacitors; a charge circuit coupled to the power source and to theplurality of output capacitors, including: a transformer consistingessentially of a single primary winding coupled to the power source anda single secondary winding coupled to the plurality of outputcapacitors, wherein the transformer is configured for charging each ofthe plurality of output capacitors; a diode for regulating a chargingpolarity for the plurality of output capacitors; a coupling means forcoupling the plurality of output capacitors in a parallel configurationduring the charging of the capacitors.
 16. The implantable medicalsystem of claim 15, wherein the coupling means decouples the pluralityof output capacitors from the parallel configuration and couples thecapacitors in a combination series configuration in response totermination of charging of the capacitors.
 17. The implantable medicaldevice of claim 15, further comprising: an input circuit for sensingcardiac signals; a microprocessor programmed to process the sensedsignals to detect a predetermined event in response to the sensedsignals; and a controller for controlling charging of the plurality ofoutput capacitors triggered by detection of the predetermined event.